CD19⁻ PC are detectable at sites of chronic inflammation and can secrete autoantibodies. (A) Synovial biopsies from 2 patients with rheumatoid arthritis were analyzed by immunofluorescence histology for the presence of CD138⁺DAPI⁺CD19⁺ and CD138⁺DAPI⁺CD19⁻ PC (1 out of 2 cases is shown). Upper and lower panels originate from the identical staining and section. (B) Kidney tissue from 2 patients after humoral rejection was digested by collagenase V, and mononuclear cells were analyzed by flow cytometry for the presence of CD19⁺ and CD19⁻ PC. PC were identified using high expression of CD38 within a wide lymphocyte gate and electronic depletion of CD3⁺ cells, CD14⁺ cells, and dead cells. In 1 of the 2 cases, CD19 expression by PC was controlled using an isotype-matched control antibody, and the PC identity was confirmed by the detection of intracellular immunoglobulin (cyt κ and λ Ab light chains). Numbers denote mean fluorescence intensities. (C) CD19⁺ and CD19⁻ BMPC from 1 SLE patient (serum anti-dsDNA antibodies, 84 U/mL) were isolated and subjected to an Elispot assay detecting dsDNA-specific IgG-secreting cells among 24 000 CD19⁺ BMPC/well and 27 000 CD19⁻ BMPC/well. Representative wells (from 14 dsDNA-coated replicate wells and 7 bovine serum albumin-coated replicate wells per PC subset to determine assay background) and assay controls are shown. After background subtraction, frequencies of dsDNA-specific PC were 0.8 and 1.3 PC per 10 000 total CD19⁺ and CD19⁻ BMPC, respectively.